Fastener driving tool



plil u, 1957 o. A. WANDl-:L 3,313,213

FASTENER DRIVING TOOL Filed May 20, 1965 5 Sheets-Sheet l 056,42 A. WANDEL 34 y 73m, /mw @an rroeA/E Ys.

April 11, 1967 o. A. WANDEL FASTENER DRIVING TOOL 3 Sheets-Sheet 2 Filed May 20, 1965 ATmQA/EYS.

O. A. vVVANDEL FASTENER DRIVING TOOL April M, lg'? 5 Sheets-Sheet 5 Filed May 20, 1965 /A/L/EA/To/Z l 055,42 A VVAA/DEL L/ mmfmm, @www @m United States Patent C) 3,3l3,2l3 FASTENER DRIVING TOL Oscar A. Wandel, Mundelein, lil., assigner to Fastener Corporation, Franklin Park, Ill., a corporation of Illinois Filed May 20, 1965, Ser. No. 457,302 4 Claims. (Cl. 91-399) This invention relates to a fastener driving apparatus and, more particularly, to a fastener driving apparatus including new and improved pneumatic piston return means.

The use of a compression spring to return the piston in a pneumatic fastener driving tool to its normal position at the end of a power stroke has largely been supplemented by pneumatic piston return means. These pneumatic piston return systems, which can avoid the failures in power tools resulting from the fatigue of the return spring and also permit a reduction in the height or the size of the tool, commonly use such techniques as continuously supplying the pressure uid to a differential piston or selectively applying compressed air below the piston following the termination of the power stroke. Some of these types of return system consume an eXcessive amount of the pressure drive fluid and require somewhat complicated valve arrangements for controlling the iiow of the piston return fluid. A piston return system that obviates many of the difficulties of the prior art is shown and described in Patent No. 3,173,340 granted Mar. 16, 1965, to Richard I-i. Doyle and Armin Fiedler.

Accordingly one object of the present invention is to provide a new and improved pneumatically actuated fastener driving tool.

Another object is to provide a fastener driving apparatus including new and improved means for pneumatically returning a piston.

Another object is to provide a fastener driving tool having new and improved means for the connection of a pressurized fiuid to the lower end of a cylinder to provide fluid means for returning a drive piston.

A further object of the present invention is to provide a fastener driving apparatus having means for conserving pressurized iiuid required to operate the tool.

Yet a further object of the present invention is to provide means for supplying pneumatic fiuid under pressure to the lower end of the cylinder to return the piston at the completion of the power stroke `without requiring complicated valving.

In accordance with these and many other objects, an embodiment of the invention comprises a pneumatically actuated fastener driving tool or apparatus having a housing with a forward head portion and a rearwardly extending hollow handle forming a compressed air or fluid reservoir. The head portion includes a generally vertically extending sleeve defining a cylinder and having an open upper end communicating with a passageway in the housing which may be sequentially exhausted to the atmosphere, or connected to the reservoir of pressurized fluid. A manually controlled valve, which may be of any suitable design, connects the passageway to the atmosphere and is operable to a position in which the connection to the atmosphere is closed and the passageway is connected to the reservoir to supply compressed air to the cylinder. This air drives a piston which is slideably mounted in the cylinder and which is connected to a fastener driving blade downwardly from a normal position adjacent its open upper end to a lower position.

In accordance with the present invention, the cylinder communicates at its lower end with an air return chamber. During the power stroke of the piston the air disposed in the cylinder below the piston is driven into the air return chamber so that the pressurized uid may be conserved. As the piston nears the completion of its 3,313,2l3 Patented Apr. il, 1967 power stroke, a check valve means in the piston is effective to connect the supply of pressurized fluid from above the piston to the air return chamber to provide a further buildup of pressurized fiuid in the air Ireturn chamber. It has been found that the conservation of the air below the piston and the driving of this air into the air return chamber has a negligible effect on the operation of the fastener driving tool. Upon completion of the power stroke of the pist-on, the open upper end of the cylinder is closed olf from the fluid reservoir, and is simultaneously exhausted to the atmosphere. At this point, the pressurized iiuid in the air `return chamber is effective to return the piston to the top of the cylinder to its normal or stati-c condition.

In a specific embodiment, the check valve in the piston is defined by an O-ring which is movable in an elongated groove against the upper and lower edges of the groove. A slot extends around the lower edge of the groove to permit the bypassing of pressurized duid around the O- ring when pressurized fluid is in the cylinder above the piston. However a second or lower O-ring prevents leakage of the pressurized uid around the piston. As the piston nears the completion of its power stroke, bleed openings between the cylinder and the air return chamber are uncovered by the lower O-ring so that pressurized iiuid is now free to bypass around the upper O-ring through the slot and enter the air return chamber through the bleed openings. Upon initial return of the piston, the upper G-ring will be driven by the air from the bleed opening against the upper surface of the groove to function as a check valve and prevent leakage of the air from the air return chamber. Moreover initial travel of the piston upwardly will drive the piston to the position where the lower O-ring passes over and past the bleed openings and leakage from the bleed openings cannot occur around or above the piston.

Many other objects and advantages of the present invention will become apparent from considering the following detailed descriptions in conjunction with the drawings in which:

FIGURE l is a fragmentary sectional View of a fastener driving tool embodying the present invention and illustrated with the piston and driver blade at the bottom of its power stroke;

FIGURE 2 is a fragmentary sectional view similar to FIGURE l, illustrating the fastener driving tool of FIG- URE 1, with the piston and driver blade intermediate its return stroke; and

FIGURE 3 is a fragmentary sectional view of the fastener driving tool of FIGURE 1, illustrating the control valve thereof in its normal or static position.

Referring now specifically to the drawings, there is i1- lustrated a fastener driving tool which is indicated generally as 10 and which embodies the present invention. The tool 1G includes a housing l2 having a forward and generally vertically extending head portion 12a and a rearwardly extending hollow handle portion 12b forming an air or fluid reservoir 14 to which a pressurized fluid, such as compressed air, is supplied in any suitable manner, as by a flexible airline. The-head portion 12a of the housing I2 includes a cavity 16 in which is mounted a sleeve forming cylinder 18. An air return chamber 19 is defined in the cavity by the cylinder sleeve 18. The lower end of the cylinder 1S is in communication with the chamber 19 and an open upper end thereof is in continuous communication with a passageway 20 formed in the housing. A manually actuated control valve assembly which is indicated generally as 22 normally connects the passageway 20 to the atmosphere through a passageway 24 formed in the hollow handle portion 12b and is operable to a position in which it connects the passageway i 20 to the fluid in the reservoir 14. The fluid admitted to the passageway 2i) enters the open upper end of the cylinder 18 and drives a power piston 26 which is slideably mounted within the cylinder 18 and which is secured to the upper end of a fastener driver blade 28 downwardly so that the lower end of the driver blade 28 engages and drives a fastener 30 supplied one at a time to a drive track 32 in a nosepiece assembly 34 by a magazine assembly indicated generally as 36.

During the downward movement of the piston 26, the air disposed within the cylinder 18 below the piston 26 is driven through a plurality of ports in the lower wall of the cylinder 18 into the return air chamber 19. When the compressed air disposed above the piston 26 is exhausted to the atmosphere under the control of the control valve assembly 22, air pressure from the return air chamber 19 is effective to act on the lower surface of the piston 26 to return the piston from its displaced position adjacent the lower end of the cylinder 18 to a normal position adjacent the open upper end thereof.

The mechanical construction of the housing 12 and the magazine assembly 36 is disclosed in detail in an application of Thomas H. Dorney, Ser. No. 326,913, filed Nov. 29, 1963, now Patent No. 3,215,324, and assigned to the same assignee as the present invention. In general, the head portion 12a of the housing 12 includes a structure defining the cavity 16 in which a flanged portion 18a of the cylinder is received with the interface between the wall of the head portion 12a and the outer wall of the flange portion 18a being sealed by a resilient O-ring 44. An upper opening in the head portion 12a is closed by a closure cap 46 secured to the housing 12 by a plurality of machine screws 48 with a resilient sealing gasket 49 interposed therebetween. A recess 50 in a depending portion 46a on the closure cap 46 carries a block or bumper 52 of resilient material that engages the upper end of the piston 26 at thetermination of its return stroke, and an annular resilient bumper 54 is disposed in the lower end of the cavity 16 to cushion the termination of the power stroke of the piston 26. The bumper 54 is held in position at the bottom of the cylinder 18 by engagement with a shouldered portion 18h formed on the cylinder 18. The lower end of the interior of the cylinder 18 is placed in communication with the cavity 16 through a plurality of peripherally spaced ports or openings 56. The chamber 19 is sealed at its lower end by a resilient O-ring 58 placed between the lower wall of the piston 26 and the inner wall of the cylinder 18.

For the purpose of admitting pressurized air to the cavity 16 there is provided a plurality of small, metered bleed openings or ports 60 in the wall of the cylinder 18 communicating with the chamber 19 and spaced above the openings 56 intermediate the length of the cylinder.

To provide means for sealing the interface between the outerwall of the piston 26 and the inner wall of the cylinder 18 there is provided a rst O-ring 62 in an annular ring channel or groove 64 in the piston wall. Additionally to provide means for selectively supplying compressed air to the cavity 16, there is provided an additional O- ring 66 in an elongated annular channel or groove 68 forming a check valve so that the O-ring 66 is free to slide in the groove 68 into engagement with the upper and lower end surfaces of the groove 68. Moreover the groove 68 is recessed to provide one or more slots 74 eX- tending from near the upper or outer edge of the groove 68 to below the lower or inner edge thereof. Thus when compressed air is supplied to the upper end of the cylinder 18, the air will pass through the interface by way of the slot 74, but when air is supplied to the lower end of the cylinder 18 below the O-ring 66, the O-ring 66 acts as a check valve and seals against the upper surface of the groove 68 thereby preventing the air from passing around the O-ring.

A thin metal sealing member 76 is provided to prevent escape of air from below the piston 26 around the driver blade 28. The driver blade 28 passes through a 4 close fitting slot 76a in the sealing member 76 to form a substantial airseal. Of course it is to be understood that in the normal operation of a pneumatically actuated fastener driving tool, compressed air may leak or bleed out around the seals but such air leakage is insignificant in the overall `operation of the fastener driving apparatus.

In order to charge the return air chamber 19 with more highly pressurized air, the bleed openings 60 are positioned to lie between the O-rings 62 and 66 when the piston 26 is in its lowermost position as indicated in FIG- URE 1.

From the above detailed description of the invention the operation of the piston return mechanism is believed clear. However, briey, it will be understood that the fastener driving tool 10 is in a normal or static position with the piston 26 in its uppermost position against the bumper 52. In this position the control valve assembly 22 is in anormal or static position discharging the space above the piston 26 to the atmosphere and blocking the flow of pressurized uid from the reservoir 14. Actuation of the control valve assembly 22 simultaneously blocks the communication of the cylinder 18 with the atmosphere and directs the pressurized uid from the fluid reservoir 14 into the upper end of the cylinder 18 driving the piston 26 and driver blade 28 downwardly through its power stroke. In the initial downward movement of the piston 26, there will be no bypass of pressurized uid around the O-rings 62 or 66 even though the O-ring 66 may be in its lowermost position within the groove 68 since the passage of such pressurized fluid is blocked by the lower O-ring 62. The downward travel of the piston 26 drives the air below the piston through the ports 56 into the return air chamber 19 and the piston 26 ends its power stroke against the lower bumper 54. At this point the lower O-ring 62 has cleared the bleed openings 66 and pressurized fluid from above the piston 26 bypasses the upper O-ring 66 through the slot 74 and enters into the return air chamber 19 through the bleed openings 60. t

Upon completion of the power stroke of the piston 26, the control valve assembly 22 is effective to shut off the communication between the fluid reservoir 14 and the top of the cylinder 18, and to exhaust the upper end of the cylinder 18 to the atmosphere through the passageways 20 and 24. As soon as the air in the upper end of the cylinder 18 is exhausted to the atmosphere, the pressurized fluid in the return air chamber 19 will be effective to drive the piston 26 upwardly to return to a normal or static position. Initial ow of air from the pressure return valve will take place through both bleed openings 60 and openings 56. The initial air through bleed opening 60 will be effective to raise the upper O-ring 66 against the upper inner surface of the groove 68 to check any further flow of air around the O-ring 66. In this manner the O- ring 66 functions as a check valve when the piston 26 is in its lowermost position, permitting the air to pass downwardly around the O-ring, but effectively blocking the return ow of air upwardly around the O-ring. The pressurized air from the return air chamber 19 4will propel the piston upwadly to its uppermost position to return the fastener driving tool 10 to its normal position. Sufficient uid will leak between the sealing means 76 and the driver blade 28 to discharge the fluid below the piston 26 to the atmosphere upon completion of the return stroke.

To operate the tool 10 by sequentially connecting the open upper end of the cylinler 18 to either the atmosphere through passageway 24 or to the :duid reservoir 14, there is provided the manually initiated control valve `assembly 22, which may be of any known suitable type, for example, of the type illustrated in the above referenced Doyle et al. patent. However, the specific valve assembly 22 illustrated in FIGURES 1 to 3 is of the type disclosed and claimed in the copending application of Howard B. Ramspeck, tiled on even date herewith.

Referring now to the drawings, the illustrated control valve assembly 22 includes a valve chamber 79 defined by an exhaust valve seat 80 carried on the housing 12 and having a downwardly and inwardly tapered opening 82 through which the passageway is normally placed in communication with the exhaust passageway 24. The opening 82 can be closed by a resilient O-ring 84 carried on the upper end of an upper valve assembly 86. A resilient valve element 88 is also carried on the upper valve assembly 86 disposed above a piston 90 forming a part of the upper valve assembly 86. The resilient valve element 88 normally closes a port or passageway 92 interposed between the reservoir 14 and the passageway 20. The piston 90 closely fits and is slidably mounted within a cylinder 94 formed in a valve body 96 that is threadingly received within a tapped opening 98 in the housing 12. A metered leakage of fluid can pass between the side wall of the piston 90 and the inner wall of the cylinder 94 through the clearance area or passageway 100.

The control valve assembly 22 additionally includes a lower valve assembly 102 telescopically received within a recess in the bottom surface of the upper valve assembly 86 and forming a cylinder 104. The lower valve assembly 102 and cylinder 104 form a chamber 106. Forming a portion of the lower valve assembly 102 is a valve piston 108 slideably received within the cylinder 94. An O-ring 110 seals the side surface of the piston 108 with the cylinder 94. A resilient valve element 112 is also carried on the lower valve assembly 102 Ion the piston 108 and engages against an annular projection or skirt 114 extending contiguous with the outer surface of the piston 90 downwardly from the lower end thereof to define a chamber 116 therebetween. An additional O- ring 118 pneumatically seals the lower valve assembly 102 against the inner wall of the cylinder 104. A metered bleed passage 120 bypasses the O-ring 118 to provide communication between chambers 106 and 116. Additionally a port 122 extends through the side wall of the cylinder 104 near its upper end to exhaust chamber 106.

A pilot valve assembly is included in the control valve assembly22 to initiate the operation thereof and to permit the assembly 22 to be operated with a minimum manually applied force. This pilot valve assembly includes an operating pin 124 that is slideably mounted within `an -axial bore formed in a member 126 that is threadingly received Within an opening 128 in the valve body 96. A trigger 129 provides for depression of the operating pin 124. An O-ring 130 seals the interface between the valve body 96 and the member 126. The upper end of the operating pin 124 is tapered and carries an O-ring 132 that operates as a valve element to control communication between the reservoir 14 and a bore or opening 134 through a passageway 136, or to selectively exhaust the bore 134 to the atmosphere through a clearance space forming an exhaust opening 138 around the operating pin 124. The bore 134 communicates with a chamber 140 formed between the lower end of the cylinder 94 and the piston 108 through passageway 142.

From the above description of the control valve assembly 22, its operation is believed clear. However, briefly, assuming the control valve assembly 22 to be in its normal or static position, as indicated in FIGURE 3, pressurized fluid from the uid reservoir 14 passes through passageway 136, over the O-n'ng valve element 132, into the bore 134, through passageway 142 and into the chamber 140 below the piston 108 carried by the lower valve assembly 102. Since the under surface of the lower valve element has a larger air-contact area from below than from above, the differential pressure is effective to drive and maintain the lower valve assembly 102 in its upper position and at the same time the upper valve assembly 86 is driven and held upwardly, as indicated in FIGURE 3. With the upper valve assembly 86 in its up position, the passageway 20 is placed in communication with the exhaust to the atmosphere through passageway 24 and opening 82. In this manner in the normal position of the tool the fluid in the upper open end of the piston 18 is exhausted to the atmosphere. Moreover any fuid in chambers 106 or 116 is also exhausted to the atmosphere through passageway 120 anl port 122. The valve element 188 seated against the port 92 is effective to block the communication between the fluid reservoir 14 and the passageway 20.

When the operating pin 124 is depressed upwardly, pressurized fluid to the chamber 140 is cut off by the seating of the valve element 132 against the opening of the passageway 136 and the air from the chamber 140 is exhausted to the atmosphere through the passageway 142, bore 134, and exhaust opening 138. Under such conditions both the upper and lower valve assemblies 86 and 102 move downwardly connecting the air reservoir 14 to the upper end of the power cylinder 18 through the passageway 20 as best illustrated in the position of FIG- URE l.

Pressurized fluid will enter into chamber 116 between the lower surface of the piston and the valve element 112 at a metered rate as determined by the size of the clearance passageway and the bleed opening 120. The pressurized fluid entering the chamber 116 will create a differential below the upper valve assembly 86 greater than that above the upper valve assembly and will be effective to drive the upper valve assembly 86 upwardly, closing off the pressurized fiuid to drive the power piston 26 and opening the exhaust port from passageway 24 to exhaust the fluid from the upper end of the cylinder 18 to the atmosphere. At the same time any fluid in the chamber 106 will be exhausted to the atmosphere through port 122 and the tapered opening 82.

As soon as the fluid from the yair reservoir 14 is cut off to the power cylinder 18, land the upper end of the power cylinder 18 is exhausted to atmosphere, the power piston 24 will be driven upwardly by the return action of the return mechanism. In the illustrated embodiment the compressed fluid from the air return chamber 19 will be discharged below the power piston 26 to drive the piston upwardly t0 its normal position in the manner heretofore described.

To complete the cycle of operation of the control valve, when the operating pin 124 is released, air from the air reservoir 14 again enters the chamber 140 below the lower valve assembly 102 and the pressure differential acting on the lower valve assembly will drive the lower valve assembly upwardly to its static position yas indicated in FIGURE 3.

Although the present invention has been described by reference to only a single embodiment thereof, it will be apparent that numerous other modifications and embodiments will be devised by those skilled in the art which will fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l1. A fastener driving tool comprising a housing having a head portion provided with `a circular opening, a sleeve forming a cylinder positioned in said opening, a cavity between said sleeve and said head portion forming an air return chamber, a fluid reservoir, fastener driving means including a piston slideably mounted in the cylinder, control valve means carried on the housing and operable sequentially to connect one end of said cylinder between a normal position exhausting to the atmosphere and an operated position communicating with said reservoir, port means extending through said sleeve adjacent the other end of said cylinder placing said other end of said cylinder in communication with said air return chamber, said piston being provided with a pair of spaced annular grooves in its outer wall, the one of said grooves closest to said one end of said cylinder being elongated, a rst O-ring in said one of said grooves and being free to slide therein into engagement with the end surfaces thereof, a recessed slot extending from near the outer edge of said one of said grooves to beyond the inner edge thereof to provide a fiuid bypass around said O-ring when said O-ring is out of engagement with t-he outermost one of said end surfaces, another O-ring in the one of said grooves closest to said other end of said cylinder and forming a pneumatic seal between said piston and said cylinder, and fluid -bleed passageway means extending between said air return chamber and said cylinder to a point in said cylinder spaced between said O-rings when said piston is adjacent said other end of said cylinder.

2. A fastener driving tool comprising a housing having a structure defining a cylinder, an air return chamber defined in said housing, a uid reservoir, fastener driving means including a piston slideably mounted in the cylinder, control valve means carried on the housing and operable sequentially to connect one end of said cylinder between a normal position exhausting to the atmosphere and an operated position communicating with said reservoir, port means connecting between the other end of said cylinder and said air return chamber, said piston being provided with a pair of spaced annual grooves in its outer wall, the one of said grooves closest to said one end of said cylinder being elongated, a rst O-ring in said one of said grooves and being free to slide therein into engagement with the end surfaces thereof, la recessed slot extending from near the outer edge of said one of said grooves to beyond the inner edge thereof to provide a uid bypass around said O-ring when said O-ring is out of engagement with the outermost one of said end surfaces, another O-ring in the one of said grooves closest to said other end and forming a pneumatic seal between said piston and said cylinder, and iiuid bleed passageway means extending between said air return chamber and said cylinder to a point in said cylinder spaced between said O-rings when said piston is adjacent said other end of said cylinder.

3. A fastener driving tool comprising a housing having a sleeve structure dening a cylinder, an air return chamber defined in said housing, a uid reservoir, fastener driving means including a piston slidea'bly mounted in the cylinder, control valve means carried on the housing and operable sequentially to connect one end of said cylinder between a normal position exhausting to the atmosphere and yan operated position connecting to said reservoir, port means extending through said sleeve structure adjacent the other end of said cylinder placing said other end of said cylinder in communication with said air return chamber, said piston being provided with an elongated groove, an O-ring in said groove and being free to slide therein into engagement with the end surfaces thereof, said piston being provided with a recessed slot extending from near the edge thereof closest to said one end of said cylinder to beyond the inner edge thereof to provide a fluid bypass around said O-ring when said O-ring is out of engagement with the outermost one of said end surfaces, and iiuid bleed passageway means extending between said air return chamber and said cylinder to a point in said cylinder spaced below said O-ring when said piston is adjacent said other end of said cylinder.

4. A fastener driving tool comprising a housing having a structure dening a cylinder, an air return chamber defined in said housing, a iluid reservoir, fastener driving means including a piston slideably mounted in the cylinder, control Yvalve means carried on the housing and operable sequentially to connect one end of said cylinder Ibetween a normal position exhausting to the atmosphere and an operated position connecting to said reservoir, port means extending through said cylinder adjacent the other end of said cylinder placing said other end of said cylinder in communication with said air return chamber, said piston being provided with a pair of spaced annular grooves in its outer wall, first and second resilient rings in respective ones of said grooves, check valve means positioned to bypass uid from said one end of said cylinder around the one of said rings nearest said one end of said cylinder, and fluid bleed passageway means extending between said air return chamber and said cylinder to a point in said cylinder spaced between said O-rings when said piston is adjacent said other end of said cylinder.

References Cited bythe Examiner UNITED STATES PATENTS 2,746,425 5/1956 Schafer 91-399 2,983,922 5/1961 J uilts 91--394 3,094,043 6/1963 Powers 91-422 X 3,205,787 9/1965 Volkmann 91--399 3,208,353 9/1965 Wandel 91-422 MARTIN P. SCI-IWADRON, Primary Examiner.

P. T. COBRIN, P. E. MASLOUSKY, Assistant Examiners. 

1. A FASTENER DRIVING TOOL COMPRISING A HOUSING HAVING A HEAD PORTION PROVIDED WITH A CIRCULAR OPENING, A SLEEVE FORMING A CYLINDER POSITIONED IN SAID OPENING, A CAVITY BETWEEN SAID SLEEVE AND SAID HEAD PORTION FORMING AN AIR RETURN CHAMBER, A FLUID RESERVOIR, FASTENER DRIVING MEANS INCLUDING A PISTON SLIDEABLY MOUNTED IN THE CYLINDER, CONTROL VALVE MEANS CARRIED ON THE HOUSING AND OPERABLE SEQUENTIALLY TO CONNECT ONE END OF SAID CYLINDER BETWEEN A NORMAL POSITION EXHAUSTING TO THE ATMOSPHERE AND AN OPERATED POSITION COMMUNIC ATING WITH SAID RESERVOIR, PORT MEANS EXTENDING THROUGH SAID SLEEVE ADJACENT THE OTHER END OF SAID CYLINDER PLACING SAID OTHER END OF SAID CYLINDER IN COMMUNICATION WITH SAID AIR RETURN CHAMBER, SAID PISTON BEING PROVIDED WITH A PAIR OF SPACED ANNULAR GROOVES IN ITS OUTER WALL, THE ONE OF SAID GROOVES CLOSEST TO SAID ONE END OF SAID CYLINDER BEING ELONGATED, A FIRST O-RING IN SAID ONE OF SAID GROOVES AND BEING FREE TO SLIDE THEREIN INTO ENGAGEMENT WITH THE END SURFACES THEREOF, A RECESSED SLOT EXTENDING FROM NEAR THE OUTER EDGE OF SAID ONE OF SAID GROOVES TO BEYOND THE INNER EDGE THEREOF TO PROVIDE A FLUID BYPASS AROUND SAID O-RING WHEN SAID O-RING IS OUT OF ENGAGEMENT WITH THE OUTERMOST ONE OF SAID END SURFACES, ANOTHER O-RING IN THE ONE OF SAID GROOVES CLOSEST TO SAID OTHER END OF SAID CYLINDER AND FORMING A PNEUMATIC SEAL BETWEEN SAID PISTON AND SAID CYLINDER, AND FLUID BLEED PASSAGEWAY MEANS EXTENDING BETWEEN SAID AIR RETURN CHAMBER AND SAID CYLINDER TO A POINT IN SAID CYLINDER SPACED BETWEEN SAID O-RINGS WHEN SAID PISTON IS ADJACENT SAID OTHER END OF SAID CYLINDER. 